Eli5: What exactly are quarks…how many are there…and is there anything smaller?
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Quarks are a type of fundamental particle under the standard model of particle physics. This is a model based on our understanding of the particles and forces in the universe, which consists of a small number of fundamental particles that can be broken down any further. So according to the model, quarks are fundamental and cannot be broken down any further.
Quarks sit on the same level as electrons and photons in terms of being fundamental, these also cannot be broken down up any further under the standard model. Quarks make up protons and neutrons (and a vast array of other constituent particles). They do so by interacting via the strong force, which has rigid rules for what combinations of quarks are allowed, based on concepts like colour charge and exchange of gluons.
The existence of quarks is necessary to explain what keeps atoms together. You might be aware that an atom is comprised of a bunch of protons and neutrons bunched together into a tight nucleus, which is orbited by negatively charged electrons. You might’ve wondered what keeps the protons in the nucleus of an atom together, given they are all positively charged and like charges will repel. Well in a nucleus two main forces compete against one another, electrostatic repulsion, and the strong force attraction. Quarks explain the strong force attraction, the strong force keeps the quarks together to form the protons and neutrons, and these particles in close proximity experience a residual strong force, quarks in adjacent nucleons interact with one another as well.
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